1. Field of the Invention
Embodiments of the invention relate to the processing of semiconductor substrates. More particularly, embodiments of the invention relate to deposition of tungsten layers on semiconductor substrates using vapor deposition techniques.
2. Description of the Related Art
The semiconductor processing industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having larger surface areas. These same factors in combination with new materials also provide higher integration of circuits per unit area of the substrate. As circuit integration increases, the need for greater uniformity and process control regarding layer thickness rises. As a result, various technologies have been developed to deposit layers on substrates in a cost-effective manner, while maintaining control over the characteristics of the layer.
Chemical vapor deposition (CVD) is one of the most common deposition processes employed for depositing layers on a substrate. CVD is a flux-dependent deposition technique that requires precise control of the substrate temperature and the precursors introduced into the processing chamber in order to produce a desired layer of uniform thickness. These requirements become more critical as substrate size increases, creating a need for more complexity in chamber design and gas flow technique to maintain adequate uniformity.
A variant of CVD that demonstrates excellent step coverage is cyclical deposition or atomic layer deposition (ALD). Cyclical deposition is based upon atomic layer epitaxy (ALE) and employs chemisorption techniques to deliver precursor molecules on a substrate surface in sequential cycles. The cycle exposes the substrate surface to a first precursor, a purge gas, a second precursor and the purge gas. The first and second precursors react to form a product compound as a film on the substrate surface. The cycle is repeated to form the layer to a desired thickness.
Formation of film layers at a high deposition rate while providing adequate step coverage are conflicting characteristics often necessitating the sacrifice of one to obtain the other. This conflict is true particularly when refractory metal layers are deposited over gaps or vias during the formation of contacts interconnecting adjacent metallic layers separated by dielectric layers. Historically, CVD techniques have been employed to deposit conductive material such as refractory metals in order to inexpensively and quickly form contacts. Due to the increasing integration of semiconductor circuitry, tungsten has been used based upon superior step coverage. As a result, deposition of tungsten employing CVD techniques enjoys wide application in semiconductor processing due to the high throughput of the process.
Depositing tungsten by conventional CVD methods, however, is attendant with several disadvantages. For example, ALD processes deposit tungsten films into vias containing high aspect ratios (e.g., 20), whereas conventional CVD processes will usually cause similar vias to “pinch-off” and not completely fill. Also, blanket deposition of a tungsten layer on a semiconductor wafer is time-consuming at temperatures below 400° C. The deposition rate of tungsten may be improved by increasing the deposition temperature to, for example, about 500° C. to about 550° C. However, temperatures in this higher range may compromise the structural and operational integrity of the underlying portions of the integrated circuit being formed. Use of tungsten has also frustrated photolithography steps during the manufacturing process as it results in a relatively rough surface having a reflectivity of 70% or less than that of silicon (thickness and wavelength dependent). Further, tungsten has proven difficult to deposit uniformly. Poor surface uniformity typically increases film resistivity.
Therefore, there is a need for an improved technique to deposit conductive layers with good uniformity using cyclical deposition techniques.